Defects In Pressure Vessels Research Articles

Weight function procedure for reduced order fracture analysis of arbitrary flaws in cylindrical pressure vessels

Fracture mechanics calculations using the finite element method can be computationally expensive, which makes them challenging to use in engineering evaluations of flaws in pressure vessels. The weight function (WF) technique is a reduced order fracture modeling method that is widely used to greatly reduce these computational costs. Its computational efficiency is essential for use in probabilistic fracture mechanics evaluations of embrittled nuclear reactor pressure vessels (RPVs), due to the large number of sampled flaws that must be evaluated. Although the WF technique is general, it is typically used only for axis-aligned flaws. Recent discoveries of off-axis flaws in operating nuclear reactors have necessitated detailed simulations of such flaws and the interactions between neighboring flaws. This paper presents a generalized WF approach applicable for analyzing arbitrary flaw geometries in thick-walled cylindrical vessels, including surface-breaking and subsurface flaws, which can either axis-aligned or off-axis, and can account for interactions with other flaws. This approach is demonstrated on representative simulations of multiple flaw geometries in a RPV subjected to transient loading conditions. In all cases, the WF technique gives good comparison with benchmark results from direct simulation with greatly reduced computational effort.

Securing integrity of high-temperature pressure boundary components in supercritical thermal plants with application of alternative design rules

Technical issues of ASME BPVC Section VIII Division 2 (hereafter, ‘ASME VIII(2)’) were investigated when it was applied to design of high-temperature pressure vessels in supercritical thermal power plants subjected to long-term operation in the creep regime. The design rules of ASME VIII(2) are generally used for the design of pressure vessels operating not only at low temperature but also at high temperature in the creep range. The potential connections of ASME VIII(2) with a number of defects in pressure vessels due to technical issues on the implicit consideration of the creep effects were investigated. For the superheater component subjected to long-term operation in the creep regime, design evaluations were conducted according to ASME VIII(2) and three alternative design rules of ASME BPVC Section III Division 5 Subsection HB, French RCC-MRx and ASME Section VIII Division 2 Code Case 2843–2 that consider creep effects explicitly. Code comparisons were made in terms of conservatism. It was shown that there does exist concern about the non-conservatism of ASME VIII(2) associated with designs subjected to long-term operation in the creep range; the issues can be addressed with application of three alternative design rules to enhance integrity of pressure vessel.

SCC evaluation of composite materials for natural gas absorber based on experimental and numerical methods

Pressure vessel is a primary barrier to gathering system and an important guarantee for normal production in the oilfield. Composite materials for pressure vessels are widely used to adapt to acidic media and temperature and pressure environment. Therefore, the stress corrosion cracking (SCC) is easily caused considering harsh working condition and multi-material structure. Pressure vessel defect due to SCC can cause material strength degradation, medium leakage and even explosion. In order to analyze the stress distribution of the whole equipment and verify the crack detection results in the field, a detailed finite elements model (FEM) of the absorber considering the composite material is established based on mechanical experiments and field data. On the other hand, a WOL specimen FEM is established according to the industry standard. Finally, twelve SCC evaluation experiments of composite materials are carried out based on the results of two established FEM, and the safety windows of 316L and SA516-70 are obtained in specific stress corrosion environments. The work presented in this paper can offer a technological basis and theoretical foundation for strength design of composite materials for pressure vessels in sour environment.

Neutron Diffraction Measurement and Numerical Simulation to Study the Effect of Repair Depth on Residual Stress in 316L Stainless Steel Repair Weld

Welding is often used to repair the defects in pressure vessels and piping, but residual stresses are generated inevitably and have a great effect on structure integrity. According to the defect size, different repair depth will be carried out, which leads to different stress state. In this paper, the effect of repair depth on residual stress in 316L stainless steel repair weld has been studied by neutron diffraction measurement and finite element modeling (FEM). The results show that the residual stresses in the deep repair are larger than those in shallow repair weld, because the deep repair involves multipass welding and brings a serious work hardening. In the weld metal, the longitudinal stress has exceeded the yield stress, and increases slightly with the increase of repair depth. In contrast to the longitudinal stress, the transverse stress is more sensitive to the repair depth. With the increase of repair depth, the transverse stress increases and even exceeds the yield strength as the repair depth is 45% of the plate thickness. At the bottom surface of the plate and heat affected zone (HAZ), both the longitudinal and transverse stresses increase as the repair depth increases. It also shows that the mixed hardening model gives the best agreement with the measurement, while isotropic and kinematic hardening models cause an overestimation and underestimation, respectively. Therefore, the mixed hardening model is recommended for the prediction of residual stresses.

The Method of the Residual Life Prediction of Pressure Vessel Defect Based on Defect Failure Rate Inflection Point

The safety assessment of pressure vessel gets increasing attentions with the more extensive application. Taking the <safety assessment of pressure vessel with defect in service> as theoretical basis accomplished the failure simulation system. Completed the simulation of failure patch. Raised the new curve of failure patch. Achieved the simulation of failure rate and found the rate inflection point. And then work out the largest dimension change and the highest stress cycle times that allowed , based on the rate inflection point put forward a method of residual life calculation of pressure vessel and the definition of safety margin.

Digital Shearography 에서 Unwrapping 이미지와 FEM 을 이용한 압력용기의 내부결함 측정

Pressure vessels in vehicle industries, power plants, and chemical industries are often affected by flaw and defect generated inside the pressure vessels due to production processes or being used. It is very important to detect such internal defects of pressure vessel because they sometimes bring out serious problems. In this paper, an optical defect detection method using digital shearography is used. This method has advantages that the inspection can be performed at a real time measurement and is less sensitive to environmental noise. Shearography is a laserbased technique for full-field, non-contacting measurement of surface deformation (displacement or strain). The ultimate goal of this paper is to detect flaws in pressure vessels and to measure the lengths of the flaws by using unwrapping, phase images which are only obtained by Phase map. Through this method, we could decrease post-processing (next processing). Real length of a pixel can be calculated by comparing minimum and maximum unwrapping images with shearing angle. Through measuring several specimen defects which have different lengths and depths of defect, it can be possible to interpret quantitatively by calculating gray level.

Analysis of Temperature Field on Infrared Active Detection of Heavy Pressure Vessels Wall Surface Defect

Infrared thermal imaging technology is a new nondestructive detection technology, which is widely used in pressure vessel detection. In this paper, three-dimensional temperature distribution model of pressure vessel with defects is established, and the influence factors on infrared detection of pressure vessel defect are researched. The influence of environmental factors, thermal excitation strength, thermal excitation time and defect size on infrared detection is analyzed. The results show that: (1) increasing thermal excitation can degrade the maximum reduced temperature when infrared radiation thermal excitation temperature is lower than 200°C, while the reduced temperature can be raised with the increased thermal excitation when the temperature is higher than 200°C. (2) the maximum comparison temperature increased first and decreased afterwards with the increasing thermal excitation time, finally reduce to the comparison temperature under steady state. (3) the influence of wind speed and surface blackness in environmental factors on infrared detection precision is greater.

A practical procedure to assess critical defects in pressure vessels subjected to fatigue loads

PSA pressure vessels are equipments used in refining industries for cleaning the hydrogen upcoming from the reforming or hydrogen generation unities. In one regular inspection, some embedded cracks were found in the entrance nozzle-head weld. Then, instead of advance its decommissioning and waste much time until the refinery purchase a new vessel, a structural stress analysis using the finite element method was performed in order to obtain the stress field at the site of the crack, considering the real loading cycle. Despite the acting load is only the internal pressure, the nozzle-head weld is a region where a complex stress state is present (bending and axial stresses). ASME VIII Division 2, Appendix 5 addresses this issue by applying a rule for multiaxial fatigue life estimation for non-proportional loading. With ASME estimated fatigue life results, it was applied the British Standard 7910 procedure to decide if the equipment can operate safely. The calculation also assesses the crack growing by using a modified bi-linear Paris law. Finally, it was computed how long the cracks would take to get to their critical size and then retire definitively the equipment.

Automatic Detection of Weld Defects in Pressure Vessels using Fuzzy Neural Network

The interpretation of possible weld discontinuities in industrial radiography is ensured by human interpreters. The types of defects are porosity, lack of penetration, shrinkage, and fracture. It is thus desirable to develop computer-aided techniques to assist the interpreter in evaluating the quality of the welded joints. Using back propagation algorithm the images of weld defects are trained. The Gaussian Mixture Model (GMM) classifier is used to classify the defects in the input image. The input image is compared with the trained image and defect is detected if defect is present. The nature of the defect is identified and the type of defect is mentioned.

Constraint of Semi-Elliptical Surface Cracks in T and L-Joints

Critical defects in pressure vessels and pipes are generally found in the form of a semielliptical surface crack, and the analysis of which is consequently an important issue in engineering fracture mechanics. Furthermore, in addition to the traditional single parameter K or J-integral, the second parameter like T-stress should be measured to quantify the constraint effect. In this work, the validity of the line-spring model is investigated by comparing line-spring J-T solutions to the reference 3D finite element J-T solutions. A full 3D-mesh generating program for semi-elliptical surface cracks is employed to provide such reference 3D solutions. Then some structural characteristics of the surface-cracked T and L-joints are studied by mixed mode line-spring finite element. Negative T-stresses observed in T and L-joints indicate the necessity of J-T two parameter approach for analyses of surface-cracked T and L-joints.

Elastic analysis of semi-elliptical axial cracks in cylinders under thermal shock using the BS 7910 framework

One of the most common and important situations involving flaws in pressure vessels is the internal thermal shock of a hollow circular cylinder with an axial semi-elliptical internal surface crack. The current approaches available to analyse this specific problem are excessively conservative or cumbersome. This paper considers the internal thermal shock problem and determines the stress-intensity factors for shallowest and deepest points for cylinders with the ratio of thickness to inner radius 0.1 and 0.25. The results of calculations are presented in a similar form to the draft standard BS 7910:1997 and thus permit numerous practical applications. These results are in many cases much less conservative than the draft standard and also reveal some important cases.

The results of a round-robin exercise on pressure vessel defect assessment : Hopkins, P. International Journal of Pressure Vessels and Piping, Vol. 42, No. 3, pp.333–351(1990)

The results of a round-robin exercise on pressure vessel defect assessment : Hopkins, P. International Journal of Pressure Vessels and Piping, Vol. 42, No. 3, pp.333–351(1990)

Direct simulation of fatigue crack growth for arbitrary-shaped defects in pressure vessels

An advanced technique has been developed by the authors to predict fatigue crack growth for longitudinal and circumferential planar defects with arbitrary shape in pressure vessels. This is based on the step-by-step integration of an experimental fatigue crack growth law at a set of points along the crack front, enabling the crack shape developed during the fatigue process to be predicted. The stress intensity factors along the crack front are calculated by a three-dimensional finite element method. Automatic regeneration of finite element models for propagating cracks designed for this technique makes the simulation technique highly efficient. In this paper, following a description of the principle of the technique, some typical crack geometries are investigated. These include external and internal surface longitudinal cracks, an embedded longitudinal crack, a twin crack configuration and two circumferential surface cracks. The results obtained are compared with both the widely used ASME XI and BSI PD6493 guidelines, and some discussion on the safe use of the two guidelines is made.

Stable crack growth of axial flaws in pressure vessels

The ductile crack growth of axial through and part-through cracks in a vessel under internal pressure has been studied experimentally to contribute to the fundamental problem whether or not and under which conditions resistance curves obtained from specimens can be transferred to large scale components. The experiments and numerical analyses are part of a research program on fracture mechanics failure concepts for the safety assessment of nuclear components. Whereas only an averaged crack extension is determined in specimen tests, the local propagation of cracks may be of main importance for surface cracks in vessels and pipes. In the present experiments, the surface cracks revealed the well known canoe shape, i.e. a larger crack extension has occurred in the axial direction than in the wall thickness direction. Two of these tests have been analysed by finite element calculations to obtain the variation of the J-integral along the crack front and the stress and strain state in the vicinity of the crack. The local crack resistance appeared to depend on the local stress state. To Predict ductile crack extension correctly, J R-curves have to account for the varying triaxiality of the stress state along the crack front.

Ductile crack growth of semi-elliptical surface flaws in pressure vessels

Experiments on ductile crack growth of some axial surface flaws in a pressure vessel have revealed the well-known canoe shape, i.e. a larger crack extension has occurred in the axial direction than in the wall thickness direction. Two tests have been analyzed by finite element calculations to obtain the variation of the J- integral along the crack front, and the stress and strain state in the vicinity of the crack. The local crack resistance depended on the local stress state. To predict ductile crack extension correctly, J R-curves have to account for the varying triaxiality of the stress state along the crack front.

The results of a round-robin exercise on pressure vessel defect assessment

A full scale fatigue test on a 3·66-m diameter, 25-mm wall thickness, 11·38-m-long pressure vessel containing defective welds has been carried out by British Gas. The prime intention of the test was to demonstrate the ability, or otherwise, of a grossly defective pressure vessel to withstand large cyclic and static loads. However, the test also provided a good vehicle for evaluating defect assessment methods. The Department of Trade and Industry awarded British Gas a grant under the Department's Support for Innovation for the above pressure vessel test. This grant was used to organise a ‘round-robin’ exercise in which five organisations conducted fatigue assessments on defects in the vessel. This round-robin and a report on the vessel test, are presented in this paper. The major conclusion of the ‘round-robin’ exercise was that the analysis of pressure vessel defects requires a high degree of skill and judgement and there is a need for a documented, generally accepted, pressure vessel defect assessment procedure.

Stable crack growth of axial surface flaws in pressure vessels

Several tests on pressure vessels with axial surface flaws have shown the well known ‘canoe phenomenon’, i.e. the ductile crack extension has been larger in axial direction than in wall thickness direction. Two of the tests performed at the BAM have been analysed by finite element calculations to obtain the variation of the J- integral along the crack front and the stress and strain state in the vicinity of the crack. It was found that the local crack resistance depended significantly on the local stress state. In order to predict ductile crack extension correctly, J R curves have to account for the varying triaxiality of the stress state along the crack front.

The presentation of results from round-robin test-block trials

Test-block trials are used to assess the capability of teams and of non-destructive testing techniques to detect and size defects which are of concern for structural integrity of specific components. In particular there have been several test-block exercises involving the ultrasonic inspection of thick-section steel as part of the continuing development of inspection techniques for reactor pressure vessels. This paper examines the presentation of results of test-block trials in general. Although defects cannot be sized unless they have been detected, we concentrate on the sizing aspect of test-block trials rather than on detection of defects alone, because detection capability in recent trials has been very good. Various parameters for presenting the results of teams' abilities to correctly size defects are examined on the basis of a model and synthesized test-block results. The model is based on ASME acceptance and rejection standards for crack-like defects in pressure vessels and these are coupled with a bivariate normal distribution for the errors that a team may make in measuring the through-wall size and length of a defect. Seven hypothetical teams are used with a spread of team abilities representative of recent test-block trials. Results are synthesized for two different test-block trials each containing 40 cracks. Assuming that all teams detect all defects allows us to concentrate on the sizing ability of the teams. Teams are ranked according to different parameters in order to demonstrate which are the better parameters to use. The mean and associated standard deviations of the errors in measured sizes are found to be the most useful ways of presenting the results. Of the single parameters considered for ranking, the best ones are those that include some combination of all four types of defect classification: correct or incorrect acceptance and correct or incorrect rejection of defects. Simple parameters such as the ratio of defects correctly rejected to the total number of rejectable defects are found to be unsatisfactory. Attempts to improve such simple parameters by weighting defects according to some measure of their distance from the accept-reject boundary are found to be unreliable, yielding arbitrary rankings dependent on the particular choice of weighting. We conclude that uniform distributions of defect parameters provide a satisfactory basis for designing test-block trials to discriminate between relatively good teams and relatively poor teams where a range of abilities is expected.

Analysis of Interaction Behavior of Surface Flaws in Pressure Vessels

The evaluation of stress intensity factors for surface flaw problems and, in particular, semi-elliptical surface cracks in cylindrical pressure vessels has been well developed using the finite element alternating method. Some of the examples presented here include the interaction effects due to multiple internal longitudinal surface cracks in cylinders as recommended for analysis in the ASME Boiler and Pressure Vessel Code (Section XI). For each crack geometry, several loading cases are considered including internal pressure and polynomial pressure loadings from constant to fourth order. By the method of superposition, the magnification factors for internally pressurized cylinders are rederived using the polynomial influence functions. These influence functions give useful information for design purposes such as in the analysis of a thermally shocked cylinder. The problem of a single circumferential crack in a cylinder is also investigated using the finite element alternating method, and a number of results for such problems are also presented here.

Evaluation of acoustic emission from pressure vessels with planar flaws

Considered in this work is the analytical modeling of plastic zones developed around planar flaws in pressure vessels. Acoustic emissions are assumed to originate from the boundary of the plastic zone. This information is used in a calculation for interpreting the data. The method of solution is illustrated via an example.